Jardin 5-Piece Plastic One Way Aquarium Fish Tank Air Check Valves, Clear/Black Package Include: 5 x Check Valve; Hole Outer Diameter: 4mm; Fit Hose Dia: 4mm; One check valve for one-way outlet, two check valves for two-way outlet, ect. Place your airline tubing between your air pump and aquarium to prevent water backflow to the airline. Protect you air pump by preventing aquarium water from back siphoning during a power outage. Please use a paperclip to poke it from Input side to Output side when the outlet inside gets stuck. Use hot water to soften the hose first, it can be connected to the check valve more easily and tightly.